Cache the category tree walk (fixes ~1s File Management page load)

_build_category_index() was the one tree-scanning function never
wrapped in the app's existing 5-minute cache - it ran fresh on every
File Management page load and Sort Unsorted scan (not a manual "this
may take a moment" action like Duplicate Courses or Storage Usage), so
the cost was invisible until it was already slow. Measured ~1.0-1.1s
consistently against the live NAS-mounted library at 161 courses,
confirmed via profiling that a single call makes dozens of iterdir()
round-trips - each one a network hop over SMB. Now shares the same
cache_get_or_compute pattern and invalidate_cache() call sites as
get_all_course_dirs(), so no new invalidation logic was needed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

offlineu_core.py

@@ -1445,54 +1445,67 @@ def _build_category_index() -> List[Dict[str, Any]]:
     courses and their non-lecture contents never show up as if they were
     categories to file things under. The Unsorted folder itself is
     excluded - it's the source, never a valid destination.
+
+    This walk visits every directory in the library and, over a
+    NAS-mounted (SMB) library, each of those is a network round-trip -
+    the same reasoning behind get_all_course_dirs()'s cache, so this
+    result is cached the same way (same 5-minute TTL, same
+    invalidate_cache() call sites already busting it - no separate
+    invalidation needed): otherwise this ran fresh on every File
+    Management page load and Sort Unsorted scan, neither of which is a
+    manual "this may take a moment" action like Duplicate Courses or
+    Storage Usage, so the cost was invisible until it was already slow.
     """
-    library_root = Path(get_library_root())
-    try:
-        unsorted_root = (library_root / UNSORTED_FOLDER_NAME).resolve()
-    except OSError:
-        unsorted_root = None
-    categories: List[Dict[str, Any]] = []
-
-    def walk(directory: Path, path_parts: List[str]):
+    def compute() -> List[Dict[str, Any]]:
+        library_root = Path(get_library_root())
         try:
-            entries = sorted(
-                (p for p in directory.iterdir() if p.is_dir() and not p.name.startswith('.')),
-                key=lambda p: p.name.lower()
-            )
-        except (PermissionError, OSError):
-            return
-        for entry in entries:
-            try:
-                if unsorted_root is not None and entry.resolve() == unsorted_root:
-                    continue
-            except OSError:
-                pass
-            if (_looks_like_course(entry) or _is_unrecognized_leaf_item(entry)
-                    or _is_media_free_subtree(entry) or _is_course_wrapper(entry)):
-                continue
-            new_parts = path_parts + [entry.name]
-            path_tokens: Set[str] = set()
-            for part in new_parts:
-                path_tokens |= _tokenize(part)
-            bonus_tokens: Set[str] = set()
-            try:
-                for child in entry.iterdir():
-                    if child.is_dir() and not child.name.startswith('.') and _looks_like_course(child):
-                        bonus_tokens |= _tokenize(child.name)
-            except (PermissionError, OSError):
-                pass
-            bonus_tokens -= path_tokens
-            categories.append({
-                'path': str(entry),
-                'relative': '/'.join(new_parts),
-                'depth': len(new_parts),
-                'path_tokens': path_tokens,
-                'bonus_tokens': bonus_tokens,
-            })
-            walk(entry, new_parts)
+            unsorted_root = (library_root / UNSORTED_FOLDER_NAME).resolve()
+        except OSError:
+            unsorted_root = None
+        categories: List[Dict[str, Any]] = []
 
-    walk(library_root, [])
-    return categories
+        def walk(directory: Path, path_parts: List[str]):
+            try:
+                entries = sorted(
+                    (p for p in directory.iterdir() if p.is_dir() and not p.name.startswith('.')),
+                    key=lambda p: p.name.lower()
+                )
+            except (PermissionError, OSError):
+                return
+            for entry in entries:
+                try:
+                    if unsorted_root is not None and entry.resolve() == unsorted_root:
+                        continue
+                except OSError:
+                    pass
+                if (_looks_like_course(entry) or _is_unrecognized_leaf_item(entry)
+                        or _is_media_free_subtree(entry) or _is_course_wrapper(entry)):
+                    continue
+                new_parts = path_parts + [entry.name]
+                path_tokens: Set[str] = set()
+                for part in new_parts:
+                    path_tokens |= _tokenize(part)
+                bonus_tokens: Set[str] = set()
+                try:
+                    for child in entry.iterdir():
+                        if child.is_dir() and not child.name.startswith('.') and _looks_like_course(child):
+                            bonus_tokens |= _tokenize(child.name)
+                except (PermissionError, OSError):
+                    pass
+                bonus_tokens -= path_tokens
+                categories.append({
+                    'path': str(entry),
+                    'relative': '/'.join(new_parts),
+                    'depth': len(new_parts),
+                    'path_tokens': path_tokens,
+                    'bonus_tokens': bonus_tokens,
+                })
+                walk(entry, new_parts)
+
+        walk(library_root, [])
+        return categories
+
+    return cache_get_or_compute('category_index', compute)
 
 
 def _bonus_token_frequency(categories: List[Dict[str, Any]]) -> Dict[str, int]: